In the manufacture of semiconductor products such as integrated circuits, individual electrical devices are formed on or in a semiconductor substrate, and are thereafter interconnected to form electrical circuits. Interconnection of these devices within an integrated circuit is typically accomplished by forming a multi-level interconnect network structure in layers formed over the electrical devices, by which the device active elements are connected to other devices to create the desired circuits. Individual wiring layers within the multi-level network are formed by depositing an insulating or dielectric layer over the discrete devices or over a previous interconnect layer, and patterning and etching contact openings such as vias. Conductive material, such as tungsten is then deposited into the vias to form inter-layer contacts. A conductive layer may then be formed over the dielectric layer and patterned to form wiring interconnections between the device vias, thereby creating a first level of basic circuitry. Dielectric material is then deposited over the patterned conductive layer, and the process may be repeated any number of times using additional wiring levels laid out over additional dielectric layers with conductive vias therebetween to form the multi-level interconnect network.
As device densities and operational speeds continue to increase, reduction of the delay times in integrated circuits is desired. These delays are related to the resistance of interconnect metal lines through the multi-layer interconnect networks as well as to the capacitance between adjacent metal lines. In order to reduce the resistivity of the interconnect metal lines formed in metal layers or structures, recent interconnect processes have employed copper instead of aluminum. However, difficulties have been encountered in patterning (etching) deposited copper to form wiring patterns. Furthermore, copper diffuses rapidly in certain types of insulation layers, such as silicon dioxide, leading to insulation degradation and/or copper diffusion through the insulation layers and into device regions.
Copper patterning difficulties have been avoided or mitigated through the use of single and dual damascene interconnect processes in which cavities are formed (etched) in a dielectric layer. Copper is then deposited into the trenches and over the insulative layer, followed by planarization using a chemical mechanical polishing (CMP) process to leave a copper wiring pattern including the desired interconnect metal lines inlaid within the dielectric layer trenches. In a single damascene process copper trench patterns or vias are created which connect to existing interconnect structures thereunder, whereas in a dual damascene process, both vias and the trenches are filled at the same time using a single copper deposition and a single CMP planarization.
Copper diffusion issues have been addressed using copper diffusion barriers formed between the copper and the dielectric layers as well as between the copper and the silicon substrate. Such barriers are typically formed using conductive compounds of transition metals such as tantalum nitride, titanium nitride, and tungsten nitride as well as the various transition metals themselves. Insulators such as silicon nitride and silicon oxynitride have also been used as barrier materials between copper metallurgy and insulative layers. More recently, silicon carbide (SiC) has been used as a copper diffusion barrier material, as well as in etch-stop layers employed during trench and/or via cavity formation.
RC delay times have also been reduced by recent developments in low dielectric constant (low-k) dielectric materials formed between the wiring metal lines, in order to reduce the capacitance therebetween and consequently to increase circuit speed. Examples of low-k dielectric materials include the spin-on-glasses (SOGs), as well as organic and quasi-organic materials such as polysilsesquioxanes, fluorinated silica glasses (FSGs) and fluorinated polyarylene ethers. Totally organic, non silicaceous materials such as the fluorinated polyarylene ethers, are seeing an increased usage in semiconductor processing technology because of their favorable dielectric characteristics and ease of application. Other low-k insulator materials include organo-silicate-glasses (OSGs), for example, having dielectric constants (k) as low as about 2.6–2.8, and ultra low-k dielectrics having dielectric constants below 2.5. OSG materials are low density silicate glasses to which alkyl groups have been added to achieve a low-k dielectric characteristic.
Conventional single and dual damascene interconnect processing typically includes the formation of via cavities through a dielectric layer, in which the via etch process stops on an etch-stop layer underlying the dielectric. A resist ashing process is then employed to remove the via etch photoresist mask, and an optional wet clean operation is then performed to remove polymers and other residual materials from the via cavity. In the single damascene case, an etch-stop layer etch process is then performed to expose the underlying structure, such as a conductive feature (e.g., silicide contact or copper feature) in a pre-existing interconnect layer. The via cavity is then filled with copper and the wafer is planarized, after which further interconnect levels may then be fabricated. In the dual damascene case, after the via ashing and wet clean operations, a trench cavity is etched, followed by another ashing operation and optionally another wet clean. Thereafter an etch-stop layer etch is performed to expose the underlying structure, and the via and trench cavities are simultaneously filled with copper and the wafer is planarized.
In the conventional single and dual damascene interconnect processes, however, the etch-stop layer etch process not only etches the etch-stop layer, but also recesses the exposed dielectric material. As a result, the interlevel dielectric (ILD) and/or intra-metal dielectric (IMD) becomes thinner. In addition, in the single damascene case, the etch-stop layer etch and subsequent cleaning steps (e.g., ashing and wet clean) often change the via profile and increase the critical dimensions (CDs) thereof. As new technologies demand ever smaller CDs in semiconductor devices, CD control becomes more important. Furthermore, the conventional via sidewalls become bowed during the etch-stop etch and intervening cleaning after the via etch process, leading to via profile distortion. In the dual damascene case, the etch-stop etch and subsequent cleaning also affect the top dielectric surface and sidewalls of the trench cavity. Consequently, the effective dielectric constant of the resulting structure can be increased. Thus, there remains a need for improved methods for fabricating single and/or dual damascene interconnect structures in semiconductor wafers by which these and other adverse effects can be mitigated or overcome, without negatively impacting production costs or cycle times.